1. Field of the Invention
The present invention relates to a secondary battery and, more particularly, to a cap assembly for a secondary battery which can enhance a seal effect and prevent electrolyte from leaking.
2. Description of the Prior Art
Generally, secondary batteries are rechargeable and compact in size, and yet has a large capacity. Well known as secondary batteries are a nickel metal hydride battery, a lithium battery, and a lithium ion(Li-ion) battery.
Particularly, the Li-ion battery uses lithium metal oxide as a positive active material and carbon or a carbon compound as a negative active material. Lithium salt dissolved in an organic solvent is used as the electrolyte via which the lithium ions move between positive and negative electrodes.
Charging and discharging operations are realized while the lithium ions are moved between positive and negative electrodes.
FIG. 5 shows a conventional Li-ion battery.
A rolled electrode assembly 1 is inserted into a can 12 filled with electrolyte, the rolled electrode assembly 1 consisting of positive and negative electrodes and a separator disposed between the positive and negative electrodes. Mounted on an upper end of the can 12 is a cap assembly 2.
The cap assembly 2 is insulated from the can 12 by an intermediate gasket 18. That is, the upper end of the can 12 is crimped on the outer periphery of the cap assembly 2 with the gasket 18 interposed therebetween.
The cap assembly 2 comprises a plate 10 provided with a safety groove 8, a current control member 6 disposed on the plate 10, and a cap cover 4 disposed on the current control member 12. Welded under the central portion of the plate 10 is a circuit breaker 16 with an insulator 14 disposed therebetween. The gasket 18 is down-skirted and crimped under the periphery of the insulator 14.
The central portion of the plate 10 that is welded to the circuit breaker 16 serves the function of disconnecting a flow of the current when internal pressure of the battery is increased and of discharging gas generated within the battery due to an abnormal chemical reaction.
To realize this, the safety groove 8, which is mechanically weak, is formed when casting the plate 10 so that, when internal pressure is overly increased, the plate can be broken at a portion where the safety groove 8 is formed.
However, although the upper end of the can 12 is crimped on a periphery of the cap assembly, since tiny gaps may be formed between the parts constituting the cap assembly, the electrolyte may be leaked through the gaps during the charging and discharging operations of the battery.
In addition, the cap assembly 2 is assembled by first forming the plate 10 using a press working process. After forming the plate, the circuit breaker 16 is first welded to the plate 10, then, after interposing the insulator 14 between the plate 10 and the circuit breaker 16, down-skirted under the periphery of the insulator 14. This causes the manufacturing process to be complicated, decreasing productivity.